Update documentation to use the built-in function `next()`

[LKirst]

Should the example in the documentation of csvpy use the built-in function next() instead of the reader.next() method because most users will be using Python 3?

https://github.com/wireservice/csvkit/blob/da9c04c1cd882d013e79575cc4c5f7de18f6babd/docs/scripts/csvpy.rst?plain=1#L47

[jpmckinney]

Good call - thanks!